jdubb@bucsf.bu.edu (jay dubb) (03/27/91)
I am posting this for a friend of mine who doesn't have access to USENET, so please respond directly to mlevin@jade.tufts.edu. I was wondering. I often see warnings on electrical equipment to not mix battery types. Can anyone explain to me why this is so (or, maybe it is not so)? I have racked my brains trying to think of a reason why the current flowing between the cells should care what type of battery it came out of, but can't come up with anything (not surprising - I am a bio major...). Anyone know? Mike Levin (mlevin@jade.tufts.edu)
raoul@eplunix.UUCP (Nico Garcia) (03/27/91)
In article <77759@bu.edu.bu.edu>, jdubb@bucsf.bu.edu (jay dubb) writes: > I was wondering. I often see warnings on electrical equipment to > not mix battery types. Can anyone explain to me why this is so (or, > maybe it is not so)? I have racked my brains trying to think of a Number of reasons: two batteries in series, for instance, will have the same current flow through them. Since different types have different amounts they can discharge, one of them will run out first and may start reversing cells. And recharge circuits for ni-cad may not be compatible with gel-cells, for example. Also, different battery chemistries have different voltages. Ni-cad typically has 1.2 volts per cell, and alkaline has 1.5. This means that ni-cads in parallel with alkaline will either be drain the alkaline and overcharge themselves, or diode protection will cause the alkaline to drain, then the ni-cads. All sorts of problems. Don't do it if you can reasonably avoid it. -- Nico Garcia Designs by Geniuses for use by Idiots eplunix!cirl!raoul@eddie.mit.edu
whit@milton.u.washington.edu (John Whitmore) (03/27/91)
In article <77759@bu.edu.bu.edu> jdubb@bucsf.bu.edu (jay dubb) writes: > > I was wondering. I often see warnings on electrical equipment to >not mix battery types. Can anyone explain to me why this is so (or, >maybe it is not so)? I have racked my brains trying to think of a >reason why the current flowing between the cells should care All batteries have a total charge limit; when the number of amp-hours the cell is good for is exhausted, the cell voltage drops (and impedance rises). Mixing battery types puts several series-connected cells together, BUT always the current flow from each of the cells is the same (Kirchoff's laws, for series connection). So, if you have a 1.2 amp-hour NiCad in series with a 2.0 amp-hour alkaline cell, the end-of-life scenario is that the 1.2 amp-hour NiCad drops to zero voltage first. Then the other cell (and the load) put current through anyhow, and the voltage drops BELOW zero. Reverse-biasing the cell is a guaranteed path to early and possibly catastrophic failure. This is why many battery packs are factory-connected cells; the matching of individual NiCad cells makes a battery that will last years (and the swapping of one cell from such a pack would not significantly prolong the life of the remaining cells; the mismatch of the new and old cells would kill the old ones early). John Whitmore
szarekw@lonexc.radc.af.mil (William J. Szarek) (03/27/91)
In article <77759@bu.edu.bu.edu> jdubb@bucsf.bu.edu (jay dubb) writes: > > I am posting this for a friend of mine who doesn't have access >to USENET, so please respond directly to mlevin@jade.tufts.edu. > > I was wondering. I often see warnings on electrical equipment to >not mix battery types. Can anyone explain to me why this is so (or, >maybe it is not so)? I have racked my brains trying to think of a >reason why the current flowing between the cells should care what type >of battery it came out of, but can't come up with anything (not >surprising - I am a bio major...). Anyone know? > > Mike Levin (mlevin@jade.tufts.edu) the internal resistance of each type of battery is differant. Under heavy loads this will cause a voltage imballance and a possible overload on the battery . . . *poof* hope this helps buzz